This invention relates to laser amplifiers, and more particularly to a laser amplifier comprising an optical fiber of dual clad design which is subjected to a Stimulated Raman Scattering (SRS) signal process by a pump laser at an arbitrary wavelength.
The principle of SRS is widely known and used in connection with laser amplifiers. In U.S. Pat. No. 5,832,006, to Rice et al., issued Nov. 03, 1998, entitled xe2x80x9cPhased Array Raman Laser Amplifier and Operating Method Thereforxe2x80x9d, the primary pump wavelength signal for the Raman process was propagated along with the final and any intermediate Raman shifted signals as a single mode in the inner core of the fiber. In that invention, rare earth doping was provided in the single mode core to provide gain for the primary pumping wavelength, which obviously needed to be within the gain bandwidth of the laser transition for the doping. Pumping for the rare earth dopant was provided by incoherent, multimode laser diodes for which the output was coupled into a second, outer core region characterized by a large cross sectional area and high numerical aperture. This fundamental dual clad fiber amplifier pumping concept was disclosed in U.S. Pat. No. 4,815,079 to Snitzer entitled xe2x80x9cOptical Fiber Lasers and Amplifiersxe2x80x9d, and has recently lead to very significant advances in the power achieved from fiber amplifiers and lasers (i.e, greater than 65 W, CW, single mode).
Using a dual clad approach for Raman fiber amplifiers heretofore, to the knowledge of the applicant, has not been undertaken. Discussion of the Raman effect in fibers has been confined to single mode fiber configurations, in which the primary pumping signal for the Raman process propagates as a single mode of the fiber, as in Rice discussed above. However, SRS is a process that occurs at a point, and any point within an illuminated volume will exhibit Raman gain. In strongly waveguided structures, the gain for a mode is obtained by integrating the gain at a point over the cross-sectional area of the mode, regardless of the nature and origin of the gain.
With present day laser systems it is difficult to generate high levels of laser output power at an arbitrary wavelength, especially if good beam quality is important. There are many applications that require high average laser power at very precise wavelengths, but these wavelengths are wavelengths that do not correspond to efficient laser transitions. Two such important applications include the use of a laser to excite the sodium layer in the upper atmosphere to produce a guidestar for adaptive optical compensation of atmospherics, and the use of precisely tuned laser sources to photodisassociate chemical compounds of specific isotopes and thereby allow separation of isotopic species [e.g., U235 from U238]. It is also extremely difficult to generate high power in a diffraction limited beam from all laser sources, especially all-electric solid state devices. In U.S. Pat. No. 5,832,006, to Rice et al., a single mode pump signal in the Raman fiber amplifier was required to achieve efficient conversion.
In one approach, in the near infrared, pulsed and CW lasers are used to pump Optical Parametric Oscillators (OPO) to provide tunable output, but at lower power levels than are desired. Unfortunately, high beam quality is required for pumping OPOs. These devices have desirable characteristics as master oscillators, but do not permit ready scaling to high power.
In yet another application, the COIL (Chemical Oxygen Iodine Laser) is highly desirable as an interim directed energy source because it is efficient and can be packaged for a variety of military applications. However, the COIL has two drawbacks for battlefield applications: 1) the wavelength is fixed and lies at the edge of a water vapor absorption band (which produces thermal blooming of the beam), and 2) the wavelength is not eyesafe (to be eyesafe, the wavelength should be greater than about 1540 nm).
In still yet another application, the output of pulsed or CW diode pumped solid state lasers can be combined in a multimode Raman fiber amplifier. While this laser emits more than 1100 W with a poor beam quality, it is difficult to achieve the desired power with the acceptable beam quality.
It is therefore a principle object of the present invention to provide a method of generating high levels of laser output power from arbitrary pump laser wavelengths using Stimulated Raman Scattering in optical fibers of a dual clad design. The amplification of the optical fibers at SRS wavelength facilitates high power, improves brightness conversion, increase the conversion efficiency, facilitates low laser pump brightness, and increases output beam quality.
It is still another object of the present invention to provide a method which uses a laser pump that is operated in a multimode fashion.
It is yet another object of the present invention to provide a method which provides high conversion efficiency at a Raman shifted wavelength.
It is still another object of the present invention to provide an apparatus for generating high levels of laser output power at an arbitrary wavelength with high conversion efficiency, with the capacity for low multimode pump signal brightness and high output beam quality.
It is yet another object of the present invention to provide an apparatus for generating high levels of laser output power by propagating SRS gain along the length of an optical fiber using a Stimulated Raman Scattering gain principle in optical fibers of a dual clad design a signal core, which can support at least a single mode signal, a Raman and a low index outer cladding.
The present invention relates to an apparatus and method for generating high levels of laser output power using pump lasers of arbitrary wavelengths using the SRS process on optical fibers of a dual clad design. The apparatus and method of the present invention supports multimode operation of pump lasers. The apparatus and method can convert the high power multimode beam to a single mode at the Raman shifted wavelength with about 95% efficiency, thus providing a significant improvement in the efficiency of a high power laser.
In a preferred embodiment of the invention, the apparatus of the present invention generates high levels of laser output power with high conversion efficiency capacity for low multimode pump signal brightness and high output beam quality using a Stimulated Raman Scattering (SRS) gain principle in optical fibers of a dual clad design. The apparatus includes a signal core which can support a single mode signal, a Raman pump core, and a low index outer cladding. The refractive index ns of the single mode core is greater than the refractive index nr of the Raman pump core, which in turn is greater than the index ncl of the outer cladding. In the preferred embodiment, the signal core can be deployed within the dual cladding which increases its capacity to accommodate multimode pump laser signals.